1. Technical Field of the Invention
The present invention relates to a variable gain amplifier and particularly to a variable gain amplifier which includes a parallel connection of a resistance and a capacitor, or a parallel connection of an inductance and a capacitor.
2. Description of the Prior Art
Cascade amplifiers are employed for amplifying radio frequency signals for the pocket telephones and information terminals. Further, the cascade amplifiers are often provided with means for obtaining variable gain.
FIG. 4 is a block diagram of a conventional variable gain cascade amplifier. The amplifier as shown in FIG. 4 comprises a cascade amplifier including common-emitter transistor Q0 and common-base transistor Q1, a variable gain unit including transistor Q2 and Q3 and resistance R2, resistance R0 for applying bias V3 to the base of transistor Q0, resistance R1 for the load resistance of the cascade amplifier, voltage supply V2 for the amplifier as a whole, voltage supply V0 (voltage supply 50) for controlling the gain of the variable gain unit, signal input terminal 10, signal output terminal 40, power supplies and coupling capacitors connected with signal input terminal 10 and signal output terminal 40, respectively.
The variable gain cascade amplifier as shown in FIG. 4 obtains the variable gain by varying V0 so as to change the route of current which flows into transistor Q0. Concretely, the amplification gain becomes low, when V0 is greater than V1, because the current flows via transistors Q2 and Q3 into transistor Q0, while the amplification gain becomes high, when V0 is smaller than V1, because the current flows via transistor Q1 into transistor Q0.
If the current of Q2 is smaller than the current of Q1, The gain becomes low, when V0 is greater than V1, while the gain becomes high, when V0 is smaller than V1.
FIG. 5 is a graph of a frequency characteristics of the conventional variable gain cascade amplifier as shown in FIG. 4. The cut off frequency at the high gain state is about 4 GHz, while the cut off frequency at the low gain state is about 1.5 GHz, as shown in FIG. 5.
As shown in FIG. 5, the cut off frequency at the high gain state is different from that at the low gain state, because the impedance of the base of transistor Q2 is high due to resistance R2 at higher frequency, and CR circuit by resistance R2 and parasitic capacitance C between the base and the collector of transistor Q2 limits the frequency characteristics. Therefore, the frequency characteristics varies, as the gain varies, in the conventional variable gain amplifier.
An object of the present invention is to provide a variable gain amplifier wherein the frequency characteristics is unchanged, when the gain is changed.
The variable gain amplifier of the present invention comprises an amplifying unit for amplifying an inputted signal and a variable gain unit for varying the gain of the amplifying unit. The variable gain unit includes a transistor and a parallel connection of a resistor and a capacitor connected with the transistor. The variable gain unit may includes a transistor and a parallel connection of a resistor and an inductance.
The impedance of the base of the transistor is lowered by the parallel connection of a resistance and a capacitor in higher frequency range.
The variable gain amplifier of the present invention are applied for various electronic appliances such as pocket telephones.
According to the variable gain amplifier of present invention, the frequency characteristics remain unchanged, because the impedance of the control electrode such as a base or a gate of amplifying transistor are made low in higher frequency range, by using a parallel connection of a resistance (or inductance) and a capacitance connected with the control electrode of the amplifying transistor.